<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 容器全屏 */
        .container {
          height: 100%;
          width: 100%;
          position: fixed;
          left: 0;
          top: 0;
          z-index: 9999;
          background-image: linear-gradient(to right, yellow, pink);
        }
        /*背景幕布-黑色半透明遮罩*/
        .overlay {
          height: 100%;
          width: 100%;
          position: fixed;
          left: 0;
          top: 0;
          background-color: rgba(0, 0, 0, 0.5);
          z-index: -1;
        }
        /*左侧菜单*/
        .sidebar {
          height: 100%;
          width: 200px;
          position: fixed;
          left: 0;
          top: 0;
          background-color: #fff;
        }
      </style>
</head>
<body>
    
    <div class="container">
        <!-- 黑色半透明幕布 -->
        <div class="overlay"></div>
        <!-- 左侧菜单 -->
        <div class="sidebar">...菜单</div>
      </div>
      <div>ggg</div>
</body>
</html>